* fix(billing): close TOCTOU race in subscription transfer, centralize stripe test mocks
* more mocks
* fix(testing): provide complete Stripe.Event defaults in createMockStripeEvent
* fix(testing): make dbChainMock .for('update') chainable with .limit()
* fix(billing): gate subscription transfer noop behind membership check
Previously the 'already belongs to this organization' early return fired
before the org/member lookups, letting any authenticated caller probe
sub-to-org pairings without being a member of the target org. Move the
noop check after the admin/owner verification so unauthorized callers
hit the 403 first.